Why is this true in this series???

Tarzan and Jane borrow $B to buy a jungle condomium at an interest rate of 9.6% pa, compounded monthly. They borrow the money on 15th September, and on the 14th day of every subsequent month, they pay an instalement of $M. Let An be the amount owing after n months have passed

Tarzan and Jane borrow $B to buy a jungle condomium at an interest rate of 9.6% pa, compounded monthly. They borrow the money on 15th September, and on the 14th day of every subsequent month, they pay an instalement of $M. Let An be the amount owing after n months have passed

Tarzan and Jane borrow $B to buy a jungle condomium at an interest rate of 9.6% pa, compounded monthly. They borrow the money on 15th September, and on the 14th day of every subsequent month, they pay an instalement of $M. Let An be the amount owing after n months have passed